I've always wondered this about forced decryption: what if you have large random files on your harddrive? (irrelevant how they got there -- you may not even know) For all intents or purposes these look like encrypted data, and if they could force you to "decrypt" them you have a problem of Kafkaesque proportions.

Is there any rule that data on your drive should be somehow sensible and decryptable to human-understandable information, using keys that you have in your head?